Dell laptop screen goes off on mains but fine on battery.

Check the easy stuff first, is it set to use an external monitor instead of the internal panel?

check screensaver setttings for display during power/battery usage, make some changes.

sometimes the settings get screwed up.

To my knowledge of laptops (I own 3) if JUST the backlight is switching off as the OP indicates, this is not a real function of a BIOS or O/S setting. It may perhaps be yet another a bug in Vista since there are so many.

My feeling as well. I'd look for something physical having to do with the power plug. Perhaps it flexes a portion of the circuit board when it's plugged in.

Easy to check, though. Simply unplug from the mains, but leave the DC plug in the back...maybe wiggle it. If it doesn't work, I'm wrong.

That's not to say that the other posters are incorrect about BIOS, power saver etc....

jak

d of

I have the same problem, although not quite as bad yet. I have found that if I wiggle my AC power cord where it plugs into the back of the laptop, I can replicate the problem. I am going to try it to see if I ge the same problem when it's not plugged into the mains. It's definately a hardware issue, at least my problem is.

Is your plug a barrel connector or that 3 pin deal? I've owned two Dells in the past 9 years and both are now in the scrap bin. Good thing I held on to the supplies though as I've used one to replace a failed supply on one of my 3 laptops. I own a Toshiba, HP and an Asus.

I've managed to repair several Dells by dismantling (using downloaded service manual) and removing and repairing the power socket. It is a bit fiddly, but seems to be a reliable repair, and cheaper - just a bit of solder and Araldite - than the Dell suggestion of a new system board. There was one which had a frayed power lead, but a bit of solder fixed that too. hth Nel